[Python-Dev] bool(iter()) changed between 2.3 and 2.4
tjreedy at udel.edu
Wed Sep 21 01:55:26 CEST 2005
"Guido van Rossum" <guido at python.org> wrote in message
news:ca471dc20509201449652f11d at mail.gmail.com...
>I just finished debugging some code that broke after upgrading to
> Python 2.4 (from 2.3). Turns out the code was testing list iterators
> for their boolean value (to distinguish them from None).
This seem unnecessarily indirect.
Why not 'it != None' or now, 'it is not None'?
>In 2.3, a
> list iterator (like any iterator) is always true. In 2.4, an exhausted
> list iterator is false; probably by virtue of having a __len__()
> method that returns the number of remaining items.
According to 2.4 News, dict iterators also got __len__ method, though I saw
no mention of list.
> I realize that this was a deliberate feature,
I presume there were two reasons: internal efficiency of preallocations
(list(some_it) for example) and letting people differentiate iterator with
something left to return versus nothing, just as we can differentiate
collections with something versus nothing.
> and that it exists in
> 2.4 as well as in 2.4.1 and will in 2.4.2; yet, I'm not sure I *like*
> it. Was this breakage (which is not theoretical!) considered at all?
Searching the gmane archive with the link given on the python site for
'iterator len', I could not find any discussion of the __len__ method
Terry J. Reedy
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